1. Field of the Invention
The present invention relates to power sources. More specifically, the present invention relates protecting a DC power source when an output of the DC power source is short-circuited.
2. Description of the Related Art
Known DC-DC converters include switch-mode DC-DC converters that use a transformer to convert voltage from one level to another. An input DC voltage of the DC-DC converter is converted to a second voltage signal by an oscillator that includes the transformer, and the second voltage signal is rectified and filtered to provide an output DC voltage. In DC-DC converters, such as DC-DC converters that include Royer oscillators, it is known to separate the primary and secondary windings of the transformer so that the oscillator enters a high-frequency running mode when the output of the DC-DC converter is short-circuited.
The high-frequency running mode of a DC-DC converter occurs when the switching of the oscillator no longer depends on saturating the transformer core, and the oscillator instead switches before the saturation point of the transformer core is reached. In normal operation, the switching frequency of a Royer oscillator is governed by the following equation:
      f    =                  V        p                    4        ×        B        ×                  N          p                ×                  A          e                      ,where Vp is the voltage at the primary winding, B is the peak flux density of the transformer core, Np is the number of primary turns on the transformer, and Ae is the effective cross-sectional area of the transformer core. However, during the high-frequency running mode of the DC-DC converter, the above equation is not followed, and the switching frequency increases to many times more than what the switching frequency is during normal operation.
In order for the oscillator to enter the high-frequency running mode, the primary and secondary windings are preferably separated in the transformer. Separating the primary and secondary windings in the transformer increases the leakage inductance of each of the primary and secondary windings and also reduces the capacitive coupling between the windings. The leakage inductances are caused by each of the primary and secondary windings having a self-inductance that is in series with a respective ohmic resistance. The capacitive coupling arises from the close spacing of the first and second primary windings. Current phase lag, which arises due to the leakage inductance, has been recognized as a possible cause of premature oscillator switching when there is an overload on the secondary winding. Accordingly, driving transistors included in DC-DC converters are often operated close to their maximum collector current (Ic) rating so that the driving transistors fall out of saturation during a short-circuit at the secondary winding, since the driving transistors are not able to meet the current demand during the short-circuit at the secondary winding.
A particular concern for known power sources is damage caused to components by overheating. Overheating of a component in a power source can be caused by excessive current flowing through the component due to a short-circuit at the output of a power source. Accordingly, various techniques have been used to protect against short-circuits at the outputs of known power sources.
Inline positive temperature coefficient (PTC) circuit elements have been used to limit currents in known power sources. The PTC circuit element, typically a thermistor, can be placed in series with the output of a power source. A PTC thermistor increases in resistance as temperature increases, which includes increasing heat within the thermistor. Accordingly, when the output of the power source is short-circuited, the power dissipation in the PTC circuit element increases due to increasing resistance in the PTC circuit element. Increasing the resistance of the PTC circuit element reduces the load of the short-circuit on the power source, helping to protect the power source from damage.
Current sensing has been used to detect short-circuits of power sources. Typically, current sensing is implemented by inserting a low-value current-sense resistor in series with the output of a power source. The voltage across the current-sense resistor is monitored because the voltage across the current-sense resistor is proportional to the current flowing through the current-sense resistor. If the voltage across the current-sense resistor rises above a predetermined level, a shutdown mechanism for the power source is activated to reduce the output of the power source, thereby helping to protect the power source from damage.
Thermal tripping has also been used to respond to short-circuits of power sources. In a similar manner to the current sensing above, a thermistor is thermally coupled to a critical semiconductor device or other component in a power source (e.g., a driving transistor). The temperature of the thermistor is then monitored and, if the temperature of the thermistor rises above a predetermined level, a shutdown mechanism can be activated to prevent any further heating and damage to the power source.
Many types of known short-circuit protection methods “latch”, which causes the short-circuit protection mechanism or power source to stop operating and remain in a non-operative state even after the short-circuit is removed. For the short-circuit protection mechanism or power source to resume normal operation, input power must be disconnected and then reconnected for the short-circuit protection mechanism to “reset” or “unlatch”.